Electrostatic earphone



April 2, 1963 w. 'r. SELSTED ETAL 3,034,229

ELECTROSTATIC EARPHONE Filed March 11, 1960 2 Sheets-Sheet 1 WALT/:12 T SELSTED GEORGE A. BPETTELL I N V EN TOR5 BY LMQ F April 1963 w. T. SELSTED ETAL 3, 8

ELECTROSTATIC EARPHONE Filed March 11, 1960 2 Sheets-Sheet 2 m'me) 35 CONDUgLlNG COATING \22 1 AUDIO INDUT WINE/2 I S'LSTED GEOPGEABPETTELL IN V EN TORJ BY LWQWL ATTORNEY J: II3 E AUDIO INPUT 3,i)$4,229 ELECTRESTATEQII EAREHQNE Walter T. Selsted, Woodsi-de, and George A. Brettell,

Redwood City, Calif., assignors to Ampex Corporatien, Redwood City, Qaiitl, a corporation of California Filed Mar. ll, 19%, Ser. No. 14,402 11 Claims. (Cl. 179l1l) The present invention relates to earphones and more particularly to an earphone having an electrostatic transducer for changing electrical energy to acoustical energy.

Numerous types of earphones have been developed, among the most common being the earphone having a ferromagnetic disc or diaphragm mounted in spaced-apart relation with respect to an electromagnet. The electromagnet is driven by an audio signal and the electromagnet and diaphragm are assembled in a unit to fit over the ear of a listener. When the earphone is in place a confined volume of air exists between the diaphragm and the listeners ear drum. This volume of air is then pulsated in response to the audio signals impressed upon the electromagnet so that the listener hears the sig nals. While recent advances in materials for earphones have served to decrease the weight and increase the sensitivity with some increase in the frequency response, such improvements are often far short of those achieved in larger and more complex loud speakers.

Another type of earphone, which is not as common as the foregoing type, is made in the form of a condenser with one metallic plate being fixed and a second deformable metallic plate mounted in spaced-apart relation to the first. By connecting the audio signal across the two plates the electrical charges built up on the plates cause the flexible plate to vary the pressure of the air volume, which pressure changes are then converted to sound by the listeners ear. While the electrical forces are developed over the whole of the flexible plate, it has been found that distortions occur because of migratory charges and that the efficiency, frequency response, and sensitivity of this type earphone are low.

In general, it has been found that persons using usual types of earphones become tired after a relatively short period and are emotionally affected. This has been found to be caused by the fact that present day earphones are acoustically sealed so that the only sound incident upon the wearers ears are those provided by the earphones. Then after a short listening period the wearer of earphones has a tendency to readjust the headset so that at least one of the earphones is partially ed the ear. A study of the human factors involved reveals that the readjustment permits background noises from the surroundings to be heard in addition to the sounds issuing from the earphones and that this action relieves the tensions built-up from the previous closed-in condition. By changing the placement tor which the earphones were designed, the frequency response of the earphones is considerably deteriorated.

It is therefore, an object of the present invention to provide a new and improved earphone for converting electrical energy to acoustical energy.

Another object of the invention is to provide an electro static earphone having an exceptional frequency response characteristic and sensitivity.

A further object of the invention is to provide an earphone wherein the air between the moving diaphragm and the wearers ear drum is maintained in an essentially air-tight state while the earphone is substantially transparent acoustically.

A still further object of the invention is to provide an electrostatic earphone wherein the moving diaphragm is acted upon by push-pull electrostatic forces relative to fixed plates disposed on opposite sides thereof.

Patented Apr. 2, 1953 ice In accordance with the present invention an electrostatic earphone is provided having two fixed conductive plates with a thin flexible diaphragm of high resistivity mounted therebetween. The two fixed plates and the diaphragm are connected in circuit relationship to provide push-pull electrostatic forces on the diaphragm to efiiciently convert electrical energy at audio frequency to acoustical energy. A conductive ring in contact with the peripheral edge of the diaphragm, and excited by a DC. potential source, results in a charge being uniformly built up over both surfaces of the diaphragm, which cannot migrate in times as short as audio signal times due to the high resistivity of the diaphragm, considered in conjunction with the capacity to the fixed electrodes.

The two fixed plates are perforated and the diaphragm is made of a material of very low mass thus permitting external sounds to penetrate to the ear drum of the listener. In assembly, an earphone pad of a soft flexible material, is incorporated at a position between the ear and the earphone proper. Thus, an essentially air-tight condition is maintained when the earphone is over the ear thus permitting good low frequency response. On the side of the diaphragm away from the ear an acoustically absorbent material in the form of a damping element may be used to provide any necessary acoustical damping. Ambient noise is, however, permitted to pass through so that the wearer remains comfortable.

Other objects and advantages will be apparent from the following description and claims considered together with the accompanying drawings, in which:

FIG. 1 is a view illustrating the manner in which the present invention may be worn by a person;

FIG. 2 is a rear view of one of the earphones according to the present invention as shown in FIG. 1;

FIG. 3 is a cross section of the invention as taken along the line 3-3 of 'FIG. 2;

FIG. 4 is a front view of the earphone of FIG. 2;

FIG. 5 is an exploded view of the invention showing the various principal elements of the earphone of FIG. 2;

FIG. 6 is a schematic diagram of one form of audio driving circuit for the earphone of FIG. 2; and

'FIG. 7 is a schematic diagram of a second form of audio driving circuit for the earphone of FIG. 2.

Referring, now, to the drawings in detail, FIG. 1 in particular, there is shown a headset 1%), as worn by a listener, and comprising two similar earphones l1 suitably mounted at opposite ends of a bowed, spring type head band 12. The earphones are adjustabiy retained by a sliding engagement between a bracket 13 pivotally mounted on each of the earphones and a respective holder 14 afiixed to the head band 12. Thus, by adjustment of the relative position of the earphones 11 with respect to the head band 12 the listener is able to wear the headset with physical comfort.

The two earphones 11 are the same and only one will be described in detail. The bracket 13, as illustrated best in FIG. 2, is U-shaped with the respective ends attached, as by soldering, in spaced-apart relation to a shaft to. Bearings 17 are provided to rota-tably receive the extended ends of the shaft 16 and are mounted in spaced-apart relation on a back plate 18 by means of brads or screws 19. The foregoing elements of the earphone 11, as described, are conventional and many other structural combinations may be used to accomplish the same results.

The operational elements of the earphone 11, in accord with the present invention, are best seen in FIGS. 3, 5 and 6 and comprise the back plate 18 and an acoustical damping element 2% such as a mat of glass fibers, a synthetic cellulose member, or any other fibrous, cloth or porous material providing some resistance to air fiow. In the present instance a porous pad of glass fibers is preferred. The elements also include a first disc 21 of a nonconduc-ting material having a conducting. coating 22 on one face away from the back plate 18, a thin diaphragm 23, on the order of .0005 inch in thickness, of a light weight material having a high resistivity. of; about 9 ohms/square, and'a second disc 24'that'ist similar to the first disc '21'with' the'con'ducting 'coating26 facing the diaphragm 23g. Forsafety reasons the back'plate"1S is'r'nade from afdiele'ctric'material,- such as hard rubber or an epoxy glass rr'iat'er'ial. Polyester 'film"materi als' of highi'streng'thfs'uch as those sold under the names Mylar by the Du Pont Corp. and Saran by the Goodyear Co. are suit able mater'ials for t'he diaphragrn 23. Each of the twodiscs Hand 24 are 'made'ofaiplastic material such: as anep'oxy glass material for strength and the conducting coatings 22' and" 26, respectively; may be silver or other conducting paint. The 'discs2ll and 24 may a-lternatively be of perforated metal sheets, without. a conducting'coating butof approximately the' sai'ne thickness.

For'assembly' of the foregoing elements there is pro"- vided' a first r'i'ng'27 having'an inner diameter substantially equal 'to"-the outer diameter of the damping element120 to "receive the same. The inner face 28 'of the ring 27 is radially 'contouredfltoprovide (see FIG. 5 particularly) an 'inner'ledge' 2-3, a land 31; a groove' 32, and an outer rim 33' with the thickness being greater at the rim. The diameter'of'the first disc 21 is selected so .that the periphery thereof rests against ledge 2,9a'nd' is suitably alfixed thereto, as by an adhesive (not shown).

Asecond ring 36 having inner and outer dimensionssubstantially equal to those of the'ifirst ring 27 has an inner Jr-ace 37,]best illustrated-in FIG. 6, with 'an'inner' ledge '38, an intermediate ledge 39, and rim 41. Ledge 3'8 and rim 41 respectively match ledge 23 and'rim 33' of the first "ring 27. 'The' diameter of the second'disc '24 is 'sub- .stantially equal to that of the first disc 21 and is suitably secured to the ledge38, as'-by an adhesive (not shown).

The inner suriace of rim 41 is rendered 'conductiveyas by a layer of conductive silver paint or evaporated co'pper 42, and the reason therefore ;will be set forth hereinafter.

I The inner surface of the ring 27' is likewise provided'with a conductive layer 34 extending" throughjthe groove 32. The otheriacef llv' of the secondjr-ing' 36'is provided with a concave portion Both the first l'andfsecond rings 27 and 3 6 are madeof a' dielectric"material, "such' as "an epoxy glass, for strength and safety.

With the elementalrelationships maintained in'the foregoing manner the two rings 27' and 36 are brought together in matching relation with the diaphragm 23 extended and clamped between the respective rims '33 and 41. An O-ring 46 of resilient material, such as rubber, is disposed within groove 32 to force the diaphragm'23 against the conductive coating 42 of rim '41. Such'assembly is then suitably secured together, as by a plurality of screws 47 extended throughapertures 48 ofthe'back plate 18, through matching'apertures 49 of the first ring 27, and into suitably threaded holes 51 of the second ring 36. The screws 47 are here chosen of a dielectric material but may be mounted instead in dielectric bushings. For comfort and to provide an air seal about the ear of the wearer, a soft ear'padSZ of toamrubber having only slight air leakage (see FIGS. 3 and 4) is provided and contoured to the concave portion 44 and to the outer dimensions of the first and second rings 27 ond lead 57 is connected to the diaphragm 23, and the third lead 58 is'connectedto the conductive coating 26 of the second disc 24. These three leads S6, 57 and 58 are suitably trained from the respective connections through channels 59 (see FIG. 3) and through a single opening 61 in the back plate '18. To protect the'connections of the leads 56, 57 and 58 from external forces, a retaining plate 63 (FIG. 2) is mounted'adjacent to'aper- 3 going structure with reference to FIG. 3.

ture 61 on the back plate 18, as by screws 64, to clamp the leads within a channel 66 of such plate.

Thus, the first and second discs 21 and 24 are respectively mounted in spaced-apart relation with the diaphragm 23 mounted in insulated relation therebetween. The diaphragm 23 is'coated with an antistatic agent, such as stannous chloride, to provide desired build-up and maintenance of charge on the diaphragm surfaces during operation. The efiect of such an agent is well known in the art. Electrically, therefore, the combination of the two discs 21 and 24 and the intermediate diaphragm 23 provide two back-to-back' capacitors having a-plate in common (diaphragm 23). Circuitry for suitably driving FIGS. 6'and 7, hereinafter.

Consider, now, the acoustical properties of the fore- The back plate 18 is perforated by a plurality of apertures 76- to pass external sound to the interior of earphone 11. The material of plug 20 has been set forth as being acoustically absorbent. However, the absorption is slightso that external soundpassed by the apertures 76 penetrates the entire structure to the listeners ear.

To permit sound to pass through the first disc 21, aper-- tures "78 are provided through the first disc 21, but at the same time care must be exercised so that the electricalproperticsas a'plate of a capacitor are not destroyed. It has been found that, by establishingthe ratio of apertures 78 to the remaining material of disc 21 at a value on the order of sixty percent, theelectrical properties are maintained and that the first disc is then substantially transparent acoustically. .the'diaphragm 23 is essentially acoustically transparent The material of because of the light mass of the diaphragm 23 sothat external sound penetrates to thesecond' disc 24, where thesame'conditions exist as set forth' for the first disc 21.

Additionally, since the second disc 24 is inthe path of the air column between the diaphragm 23 and the listeners ear drum, the effect upon the frequency response must beconsidered. Here again, it has been found that,-by providingapertures 31 through the second disc 24 so that the ratio of apertures to material of the disc is about sixty percent, all requirements are suitably met. The plug 2! causes less than 2 db of attenuation in a -preferred arrangement, so that virtually no diminution of ambient noise is apparent. The O-ring 46 is seated against the conducting coating 34 in the groove 32 of the first ring 27. Thus little potentialexists across the i to a negligible level.

'0-ri ng 46 and a full polarizing potential'can be maintained on the diaphragm 23. Withthis arrangement, the resonant peaks which are otherwise aptto 'arisearekept I The resonant peaks'would arise because of the "presence of the pressure region of air,

, next to the ear of a wearer, the mass 'o'fthe'diaphragm and the effects of the apertures. The acoustically absorbent material, however, introduces an attenuating factor which is sufiicient to dissipate these resonances. While the ear pad 52 is largely air tight, a small leakage is'desirable for theelimination of what may be considered second order resonance e'fliects. If no internal to external air paths exist within the ear 'pad 52, a smallaperture may be provided for this purpose.

To suitably drive the ear-phone 11 a source of audio irequency signals (not shown)- is con'nected to terminals K101 and 102, respectively, 'ofan'interconnecting circuit 103 as shown in FIG. 6. The signals at terminalsllll and 102 are connected to one winding 1tl5 of an audio transformer 106 to provide a'series circuit therewith.

Earphone leads 56 and 58, as respectively connected to the'two discs 21 and 24, are connected to end leads 107 and 108 of a second winding 109 of transformer 106.

' Earphone lead 57, as connected to the diaphragm 23, is

connected through a current limiting resistor 111 and polarizing bias supply 112 to a variable intermediate tap 113 of the. second winding 1G9.

Thus, in operation the potential of the bias supply 112 provides polarization of the two capacitors formed by the two discs 21 and 24 relative to the diaphragm 23. Audio signals at the terminals 101 and 102 are then translated to push-pull signals at the second winding it for application to the two discs 21 and 24 and diaphragm 23. The high resistivity, on the order of ohms/ square, of the diaphragm 23 and the value of the limiting resistor, about 5 megohms, results in substantially no flow of current in the diaphragm circuit and a constant charge exists on the diaphragm. The diaphragm 2,3 is maintained under tension by the associated structure.

Therefore, as the potential of the respective discs 21 and 24 varies in push-pull response to the audio signal, electrostatic forces act upon the thin diaphragm 25 to move the diaphragm and thus vary the pressure of the air in the pressure region defined by the volume between diaphragm and ear drum. The differences of pressure are then added as sound to the external ambient sound penetrating the earphone 11 at the listeners ear. The conductive coating 42 on rim 41 is, as described above, in contact with the peripheral edge of diaphragm 23 and this results in a constant D.C. voltage for all parts of the diaphragm, in the absence of a signal voltage. The effect of the high resistivity of the coating is to prevent momentary migration of charge. The presence of the anti-static coating also assists in these respects. The time of charging of the diaphragm 23 is effectively made large relative to the lowest audio frequency to be generated. Accordingly, there is a substantially constant direct current potential on the diaphragm 23, and also a substantially constant total charge relative to alternating current variations. These factors contribute to the reduction of harmonic distortion in the device. In addition, the existence of the pressure region permits the diaphragm 23 to be relatively close to the discs 21, 24. Because of the pressure region, wide movements of the diaphragm 23 are not needed to generate sound at low frequencies. The close spacing, in turn, improves the electrostatic coupling between the polarized diaphragm and the associated discs 21, 24.

A second and preferred audio driving circuit is illustrated in FIG. 7 with a source (not shown) of audio frequency signals connected between a grounded terminal 12.1 and a second terminal 122. The signals at terminals 121 and 122 are developed across a resistor 123 connected therebetween and applied from control grid to cathode of a triode amplifier tube 124. The referenced cathode circuit of tube 124 comprises a resistor 126 connected directly from cathode to ground. To supply an operating potential a 700 volt source (not shown) has a positive terminal 127 connected to a resistor lid-capacitor 129 filter combination .131, which, in turn, is connected through a dropping resistor 13-2 to the anode of tube 124,.

A coupling capacitor 133 applies audio signal variations at the anode of tube 124 to the control grid of a pentode amplifier tube 134 with a grid resistor 136 connected to ground. The suppressor grid of tube 13% is directly connected to the cathode, which, in turn, is connected to ground through a resistor 137. Screen grid potential for tube 134 is applied by a direct connection to the filter 131 and operating potential is applied from positive terminal 127 to the anode through a dropping resistor 138.

One earphone lead 58, in this instance, is grounded, thereby applying a ground potential to the second disc 24. Another earphone lead 57 is supplied a polarizing potential from the positive terminal 127 through a current limiting resistor 141. Audio signal variations are then applied to the remaining earphone lead 56 by a coupling capacitor 142 connected to the anode of tube 134.

The foregoing circuit operates in the normal manner of a two stage cascade amplifier having a substantially constant frequency response over the audio frequency range; however, when driving the earphone 11 the frequency response of the combination falls oif at the higher frequencies. To increase the high frequency response of the combination a feedback network 151 is coupled between the earphone lead 56 and the cathode of tube 124. Such network 151 comprises a series resistor 152 connected between earphone lead 56 and the cathode of tube 124, with a series resistor 153 and capacitor 154 in parallel with cathode resistor 126 of tube 124. It is also to be noted that the foregoing circuit provides a high impedance and low voltage input to the earphone 11.

With the high frequency compensation provided by the feedback network 151, the amplifier circuit of FIG. 7 has a substantially constant frequency response over the lower and intermediate frequencies of the audio range, but increases over the higher frequencies. Thus, when the circuit drives the earphone 11, the inherent decrease of response at the higher frequencies of the earphone is compensated by the increased high frequency response produced by the feedback network 151.

The operation of the circuit of FIG. 7 and earphone 11 combination is substantially the same as described with respect to FIG. 6, but with the first disc 21 supplied with an audio signal, the diaphragm 23 biased with a DC. polarizing potential, and the second disc 24- maintained at DC. ground potential. Although the excitation is provided to a single one of the discs, the circuit as shown will be recognized to be the electrical equivalent of a push-pull excited device. As before, the frequency response of the earphone 11 is determined by the thickness of diaphragm 23 (.0005 inch), the approximately sixty percent ratio of the apertures of discs 21 and 24 to material thereof, size of the air column between the dia phragm 23 and listeners ear drum, and the amount of damping materials. In connection with the last factor it is to be noted .that the attenuation (about 2. db) of such material provides a dampening of resonant peaks caused by resonances in the various masses, both air and material.

Thus, there has been described an efiicient electrostatic earphone having improved sensitivity and frequency response over the audio frequency range. Also, the earphone is structurally arranged to introduce sound from the wearers surroundings to enable comfontable use by the listener for a longer period of time than was previously comfortable. Two of the earphones 11 may be openated in parallel from the circuits described to provide a headset as shown in FIG. 1 of the drawings, or two such amplifiers can be used, one connected to each earphone, for binaunal use.

What is claimed is:

1. An earphone including the combination of a thin resistive diaphragm, electrostatic plate means adjacent the diaphragm, the combination of diaphragm and plate means being substantially acoustically transparent, sealing means encompassing the diaphragm and plate means and defining a principally enclosed pressure region about the ear of a wearer, said sealing means having a portion substantially acoustically transparent to permit passage of external, ambient sounds through the earphone, and circuit means coupled to the diaphragm and plate means for electrostatically vibnating the diaphragm.

2. An earphone including the combination of a thin resistive diaphragm providing relatively slow migration of charges thereon, electrostatic plate means adjacent the diaphragm, the combination of diaphragm and plate means being substantially acoustically transparent to permit passage of external, ambient sounds, sealing means encompassing the diaphragm and plate means and defining a pressure region about the ear of a wearer, circuit means coupled to the diaphragm and plate means for eleotnostatically driving the diaphragm in push-pull relation, and acoustic absorbing material disposed adjacent the diaphragm and plate means in the path of ambient sounds passing therethrough.

3. In an electrostatic earphone, the combination comfixed and conductive plate mounted parallel'to afirst side of said diaphragm in spaced apart relation, a second tfixed and conductive plate. mounted parallel to the secone side of said diaphragm in spaced-apart relation, 'both having a plurality of apertures; means connected between said first and second plates for impressing an audio frequencysignal therebetween, means coupled between the means for impressing'an audio frequency signal and the diaphragm forapplying a polarizing bias to the diaphragm, said diaphragm having a conductive ring-at the periphery, and enclosing means disposed about theperiphcry of said diaphragm and said first and second'plates to define a pressure regionres'ponsive to ambient sounds 'as Well as sounds generated'by. electrostatic excitation of the diaphragm, saidenclosing means including means substantially acoustically'transparentto permit passage "of external, ambient sounds.

4. In an electrostatic earphone, the combination comprising a thin flexible 'di'aphr'agmof high' resistivity 'substantially equal to ohms/square, a first fixed dielectric disc having ta'conductive coating on one face mounted in spaeed apart relation with and parallel to one side of said diaphragm with said coating toward said diaphragm, a second fixed dielectric disc having a conductive coating on one face mounted in: spaced-apart relation with and parallel to a second side of said diaphragmwith said coating toward said diaphragm, both of said discs having a plurality of uniformly'distribu ted apertures withsuch apertures occupying substantially sixty percent of the surface thereof, means connected between said first and second 'discs for impressing an audio frequency signal therebetween, means coupled between the means for impressing an audio frequency-signaL-and the diaphragm forsap'plying a-polarizing 'biasto the diaphragm, and

enclosing meansdisposed about .the periphery of said diaphragm and said'tfirst and second discs to define a 1 pressure region responsive to ambient sounds as well as sounds generated by electrostatic excitation of the diaphragm.

5.- An electrostatic earphone comprising a substantially ed to conform to a wearer andprovide "a pressu-rable region, and damping means coupled-to the enclosing means "in close relation-to-the diaphragm, the damping means being acoustically absorbent.

' 6. In an electrostatic earphone, the combination cornprising athin flexible diaphragm of amaterial substantially transparent acoustically and having a' high resistivity substantially equal to 10 -ohms/square, a'first fixed conductive disc mounted in spaced apart relation with and parallel to one side of said-diaphragm, a second fixed condue- -tive disc mounted in spaced-apart relation with and parallel to a second side of-said diaphragm, each ofsaid discs having a plurality of uniformly distributed apertures with such apertures occupying substantially sixty percent of the surface thereof, enclosing means aboutsaid diaphragm and said discs, said enclosing-means internally supporting said diaphragmand discs and defining therewith a pressure region at the ear of a wearer, acoustic absorbing means mounted adjacent said diaphragm and said discs, and circuit means interconnected between saiddiaphragm and :8 discs for applying an audio frequency signal to establish push-pull electro-static forces at said diaphragm.

7.. The combination of claim 6 wherein said circuit 'rnoans is further defined as an audio frequency transformer having one winding connected across a source of audio frequency signal and a second winding connected between said discs with an intermediate tap of'said second winding connected to said diaphragm rthrough apol'arizing bias.

8. The combinatien'of claim 6-whereinsaid circuit means is further defined as a cascade amplifier having an input and an output with the input connected to a source of audio frequency signal and the output connected between said first and second discs, said diaphragm being connected to said discs through a source of polarizing bias, the amplifier including means for compensatingfor frequency sensitive response in the earphone structure.

9. In an electrostatic earphoneof the type having a thin high resistance diaphragm disposed between two perforated plates with audio signal means coupled thereto for driving said diaphragm,- the combination comprising an annular housing 'for mounting said diaphragm and plates, said housing having an opening parallel to said diaphragm and plates for communication of audio sound to the ear of a listener,- a perforated back plate mounted on said housing, a plug mounted within said housing to close the perforations of said back plate, said plug being substantially transparent acoustically to pass external sounds to the region of said diaphragm and having a degree of sound absorbent damping characteristic to dissipate internal audio sound resonances, said diaphragm be- 'ing substantially transparentacoustically -to pass andadd external sounds to the audio sound at the ear of the decreasing firequency responseof the earphone structure.

.1 1. The combination of claim 9 wherein said diaphragm is further characterized as having a conducting peripheral ring for uniformly distributing a charge over the surfaces of the diaphragm as provided by a source of bias.

References Cited in the file of this patent UNITED STATES PATENTS 1,586,140 Bonnette "'May 25, 1926 1,622,039 Lee 'M-ar. 22, 192.7 1,631,583 .Depew June 7, 1927 1,646,628 Nolen Oct. 25,1927 1,760,896 Chilowsky'et al June 3,1930 1,760,920 Stronrberg June 3, 1930 1,764,008 Crozier June '17, 1930 1,930,518 High 'Oct. 17, 1933 2,092,762. Kroger Sept. 14, 1937 2,110,908 Hartnraun Mar. 15, "1938 2,153,044 Jarna'k Apr. 4, 1939 2,200,604 Hicks -L May 14, 1940 2,509,310 M'oreland May 30, 1950 2,603,724 Kettler July 15, 1952 2,622,159 Herman Dec..l6, 1952 OTHER REFERENCES Journal I.E.E., August 1957; The Electrostatic Loudspeaker,-by D; TIN. Williamson, pp.460463.

Radio' B'roadcash Decernber 1929; Electrostatic Loud Speakers, by Sommers ct al.; page 107. 

1. AN EARPHONE INCLUDING THE COMBINATION OF A THIN RESISTIVE DIAPHRAGM, ELECTROSTATIC PLATE MEANS ADJACENT THE DIAPHRAGM, THE COMBINATION OF DIAPHRAGM AND PLATE MEANS BEING SUBSTANTIALLY ACOUSTICALLY TRANSPARENT, SEALING MEANS ENCOMPASSING THE DIAPHRAGM AND PLATE MEANS AND DEFINING A PRINCIPALLY ENCLOSED PRESSURE REGION ABOUT THE EAR OF A WEARER, SAID SEALING MEANS HAVING A PORTION SUBSTANTIALLY ACOUSTICALLY TRANSPARENT TO PERMIT PASSAGE OF EXTERNAL, AMBIENT SOUNDS THROUGH THE EARPHONE, AND CIR- 